When im in full screen mode, my fps is really bad (60- 100). When i enter the very small window mode, i get about 240 - 1200. There is no fix for this even on youtube and ive tried everything. I think it is that my laptop screen is 60hz, and i need 144hz to play with more fps because v sync wont turn off. Making the window bigger will have the same effects as making it fullscreen.
this is common for any game. a smaller window is essentially playing in a smaller resolution, which will obviously lead to better framerates because it's less intense on the gpu/cpu (depending on the game and how it renders graphics)